1. Field of the Invention
The invention relates to infrared energy emitters having a filament within a tubular envelope, and more specifically to infrared emitters further having an external sheath surrounding the envelope.
2. State of the Prior Art
Infrared emitters provide radiant heat in numerous applications. For instance, they are the preferred heat source for drying paints applied to metal surfaces, including solvent based paints, water based paints, and powder paints. They also provide heat for environmental test chambers and many industrial processes.
Typically, an infrared emitter comprises a slender tubular quartz envelope containing an elongated coiled filament that extends through the envelope and connects to lead-in conductors at opposite ends of the envelope. Infrared emitters may be provided in a variety of designs depending upon the desired wavelength and power density of emitted energy.
Infrared radiation emanates from the filament in all directions in a spherical pattern, and thus the power of the radiant energy decreases in proportion to the square cube of the distance from the emitter. In general, infrared emitters are employed to heat a particular object, such as a car body in a paint curing process. Only the energy which actually strikes the object is transferred to the object as heat energy, and of the energy which strikes the object, a portion will be reflected, a portion will be absorbed, and depending upon the object, a portion may be transmitted through the object. Only the radiant energy which actually strikes the object and is absorbed provides heat to the object. The remaining radiant energy is simply lost to the environment, thereby reducing the overall energy transfer efficiency from the infrared emitter to the object to be heated. Of course, some of the nonabsorbed radiant energy may heat the atmosphere in which the object to be heated resides, and thus be transferred to the object by convection and conduction. However, this effect is typically, either undesired or negligible.
To improve the radiant energy transfer efficiency, the radiant energy leaving the emitter is generally focused in some manner toward the object to be heated. For instance, the infrared emitters are often employed within an enclosed chamber having reflective walls. Thus, energy not directly passing from the infrared emitter to the object and absorbed by the object, continues to be reflected off of the surfaces of the chamber until it strikes the object, escapes from an opening in the chamber or dissipates through inefficiencies in the reflectors. In most applications, more direct focusing of the radiant energy greatly improves the overall transfer efficiency. For instance, in some applications, external elongated reflectors adjacent the infrared emitters focus the emitted radiant energy in the direction of the object to be heated.
In many applications, infrared emitters are used in environments where cleanliness is essential and the heating chamber must be kept free of particulate matter. Flat walls in a heating chamber are much easier to clean and accumulate less dust than walls forming external reflectors for the infrared emitters. External reflectors that are not incorporated into the chamber walls also tend to accumulate dust and are difficult to clean.
A gold reflective coating on the outer surface of the infrared emitter can form an integral reflector. Infrared emitters with reflective gold coatings, used in a chamber with flat reflective walls, improve cleanliness in the heating chamber environment. The flat chamber walls do not tend to accumulate dust and clean easily. Additionally, there are no external reflectors to accumulate dust and be cleaned. An additional advantage of reflective coatings is reduced expense versus external reflectors. Thus, it can be appreciated that the gold reflective coating provides energy efficiency and cleanliness at a reasonable cost, making the gold reflective coating a highly desirable feature. However, the gold reflective coating places certain restrictions upon the infrared emitter design.
The emitter envelope absorbs a small portion of the infrared energy. If present, reflective metal coatings, while highly reflective, absorb a portion of the infrared radiation and become heated. Also, some of the filament's heat transfers to the emitter envelope through conduction and convection to heat the emitter envelope to high temperatures. Air tight end seals at the ends of the filament seal the envelope around the filament. Typically, temperatures above 650.degree. F. damage or destroy the seals, placing a practical upper limit upon the temperature of the envelope. Further, a gold metal reflective coatings may simply vaporize off of the surface of the envelope if heated to too high of a temperature.
External requirements may also affect the temperature requirements of the envelope. For instance, when the emitters are operating in a combustible atmosphere, it is extremely important to keep the envelope operating temperature to a minimum. For instance, the National Fire Protection Association's National Electric Code, which has been adopted by many communities as the local electric code, requires a maximum surface temperature of no more than 329.degree. F. in certain organic dust filled atmospheres. Standard T3 tungsten filament infrared emitters are rated for a 392.degree. F. minimum surface temperature.
Both the wavelength and the power density of the emitted infrared energy affect the envelope temperature, with the power density the most influential factor. Thus, the power density of the emitter is limited by the design of the infrared emitter and by the operating environment. The power density, of tungsten filament infrared emitters is typically 100 watts/lineal inch of filament length. Higher power densities adversely affect the end seals and reflective coatings. Power densities are further limited in many explosive atmospheres.